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[Pramod, 4(10): October, 2015]
ISSN: 2277-9655
(I2OR), Publication Impact Factor: 3.785
IJESRT
INTERNATIONAL JOURNAL OF ENGINEERING SCIENCES & RESEARCH
TECHNOLOGY
A REVIEW PAPER ON SOLAR THERMAL ENERGY STORAGE SYSTEM
Mr. Ladgaonkar Pramod.S.*, Mr. Sandage Rahul.N.
Lecturer, Ashokrao Mane Polytechnic Vathar tarf Vadgaon
ABSTRACT
Solar thermal energy can be stored by three different ways, namely 1) Sensible heat storage (SHS), 2) Latent heat
storage (LHS), and 3) Thermo-Chemical storage (TCS). The SHS refers to the energy systems that store thermal
energy without phase change. The SHS occurs by adding heat to the storage medium and increasing its temperature.
Heat is added from a heat source to the liquid or solid storage medium. Heating of a material that undergoes a phase
change (usually melting) is called the LHS. The amount of energy stored in the LHS depends upon the mass and latent
heat of the material. In the LHS, the storage operates isothermally at the phase change of the material. Lastly,
comparison of storage. Thermo-chemical storage (TCS) using chemical reactions to store and release thermal energy.
KEYWORDS: Thermal energy storage (TES), sensible heat storage (SHS), latent heat storage (LHS)
INTRODUCTION
Today we are face two most pressing challenges addressing our expanding energy needs and reducing our greenhouse
gas emissions from the use of fossil fuels. Solar energy offers a promising solution to both challenges because of its
abundance and lack of greenhouse gas emissions. However, a transformation from fossil fuels to solar energy requires
efficient and cost effective processes to collect, store, and transport our intermittent source of energy.
Developing efficient and inexpensive energy storage devices is as important as developing new sources of energy.
The thermal energy storage (TES) can be defined as the temporary storage of thermal energy at high or low
temperatures. Energy storage can reduce the time or rate mismatch between energy supply and energy demand, and it
plays an important role in energy conservation. Energy storage improves performance of energy systems by smoothing
supply and increasing reliability. For example, storage would improve the performance of a power generating plant
by load leveling. The higher efficiency would lead to energy conservation and improve cost effectiveness. Some of
the renewable energy sources can only provide energy intermittently. Although the sun provides an abundant, clean
and safe source of energy, the supply of this energy is periodic following yearly and diurnal cycles; it is intermittent,
often unpredictable and diffused. Its density is low compared with the energy flux densities found in conventional
fossil energy devices like coal or oil-fired furnaces. The demand for energy, on the other hand, is also unsteady
following yearly and diurnal cycles for both industrial and personal needs. Therefore the need for the storage of solar
energy cannot be avoided. Otherwise, solar energy has to be used as soon as it is received.
Thermal energy storage (TES) includes a number of different technologies. Thermal energy can be stored at
temperatures from -40°C to more than 400°C as sensible heat, latent heat and chemical energy (i.e. thermo-chemical
energy storage) using chemical reactions. Thermal energy storage in the form of sensible heat is based on the specific
heat of a storage medium, which is usually kept in storage tanks with high thermal insulation. The most popular and
commercial heat storage medium is water, which has a number of residential and industrial applications. Underground
storage of sensible heat in both liquid and solid media is also used for typically large-scale applications. However,
TES systems based on sensible heat storage offer a storage capacity that is limited by the specific heat of the storage
medium. Phase change materials (PCMs) can offer a higher storage capacity that is associated with the latent heat of
the phase change. PCMs also enable a target-oriented discharging temperature that is set by the constant temperature
of the phase change. Thermo-chemical storage (TCS) can offer even higher storage capacities. Thermo-chemical
reactions (e.g. adsorption or the adhesion of a substance to the surface of another solid or liquid) can be used to
accumulate and discharge heat and cold on demand (also regulating humidity) in a variety of applications using
different chemical reactants. At present, TES systems based on sensible heat are commercially available while TCS
and PCM-based storage systems are mostly under development and demonstration.
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© International Journal of Engineering Sciences & Research Technology
[803]
[Pramod, 4(10): October, 2015]
ISSN: 2277-9655
(I2OR), Publication Impact Factor: 3.785
METHODS OF THERMAL ENERGY STORAGE
Fig No 1 Diffrent types of thermal storage of solar energy
Sensible heat storage.
Heating a liquid or a solid, without changing phase: This method is called sensible heat storage. The amount of energy
stored depends on the temperature change of the material.
𝑇2
E = m∫𝑇1 𝐶𝑝𝑑𝑇
(1)
Where
m is the mass of material used for heat store
Cp the specific heat of mass at constant pressure.
T1 and T2 represent the lower and upper temperature levels between which the
storage operates.
(T – T ) is referred to as the temperature swing.
2
1
a) Liquid Storage Media
With its highest specific heat water is the most commonly used medium in a sensible heat storage system. Most solar
water heating and space-heating systems use hot water storage tanks located either inside or outside the buildings or
underground. Water storage tanks are made from a variety of materials like steel, concrete and fiberglass. The tanks
are suitably insulated with glass wool, mineral wool or polyurethane. If the water is at atmospheric pressure, the
temperature is limited to 100°C. It is possible to store water at temperature a little above 100°C by using pressurized
tanks. Other liquid storage media are tabulated below
Fluid Type
Table No.2 Properties of liquid media
Temp
Density
Heat Capacity
3
Range.(=oC)
(J/kg.K)
Kg/m
Thermal
Conductivity(W/m.K)
Water
-
0 to 100
1000
4190
0.63 at 38°C
Dowtherms
Oil
12 to 260
867
2200
0.112 at 260°C
Sodium
Liquid salt
100 to 760
960
1300
67.5
Ethanol
Organic liquid
Up to 78
790
2400
-
storage
Medium
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© International Journal of Engineering Sciences & Research Technology
[804]
[Pramod, 4(10): October, 2015]
ISSN: 2277-9655
(I2OR), Publication Impact Factor: 3.785
b) Solid Storage Media
Energy can be stored in rocks or pebbles packed in insulated vessels. This type of storage is used very often for
temperatures up to100°C in conjunction with solar air heaters. It is simple in design and relatively inexpensive.
Typically, the characteristic size of the pieces of rock used varies from 1 to 5 cm. An approximate rule of thumb for
sizing is to use 300 to 500 kg of rock per square meter of collector area for space heating applications. Rock or pebblebed storages can also be used for much higher temperatures up to 1000°C.Magnesium oxide (magnesia), aluminum
oxide (alumina) and silicone oxide are refractory materials, and they are also suitable for high-temperature sensible
heat storage. Bricks made of magnesia have been used in many countries for many years for storing heat. Below, the
properties of solid media storage are listed
Medium
Density
3
(kg/m )
Table No.2 Properties of solid media storage
Specific
Heat Capacity
Thermal
-6
Heat
Conductivity
ρcx10
(J/kg.K)
(W/m.K)
3
(J/m .K)
896
2.4255
204 at 20°C
Thermal Diffusivity
6
2
α = k/ρc 10 (m /s)
Aluminum
2707
84.100
Brick
1698
840
1.4263
0.69 at 29°C
0.484
Stone, granite
2640
820
2.1648
1.73 to 3.98
0.799-1.840
Stone, limestone
2500
900
2.2500
1.26 to1.33
0.560-0.591
LATENT HEAT ENERGY STORAGE
Sensible heat storage is relatively inexpensive, but its drawbacks are its low energy density and its variable discharging
temperature. These issues can be overcome by phase change materials (PCM)-based TES, which enables higher
storage capacities and target oriented discharging temperatures. PCM is the heart of the LHS system
Working of a PCM system
Initially, the PCM is in solid state. As the environmental temperature rises, it absorbs energy in the form of sensible
heat. When ambient temperature reaches the melting point of the PCM, it absorbs large amounts of heat at an almost
constant temperature. This continues until all the material is transformed to the liquid phase. In this way, heat is stored
in a PCM and the temperature is maintained at an optimum level. When the environmental temperature around the
liquid PCM falls, it solidifies again, releasing its stored latent heat. Thus, the managed temperature again remains
consistent.
Fig No 2 Classification of Phase change materials
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© International Journal of Engineering Sciences & Research Technology
[805]
[Pramod, 4(10): October, 2015]
ISSN: 2277-9655
(I2OR), Publication Impact Factor: 3.785
Organic phase change materials
a) Paraffin’s
Paraffin wax consists of a mixture of mostly straight chain n-alkanes CH3–(CH2)–CH3. The crystallization of the
(CH3)- chain release a large amount of latent heat. Both the melting point and latent heat of fusion increase with chain
length. Paraffin qualifies as heat of fusion storage materials due to their availability in a large temperature range. Due
to cost consideration, however, only technical grade paraffin’s may be used as PCMs in latent heat storage systems.
Paraffin is safe, reliable, predictable, less expensive and non-corrosive. They are chemically inert and stable below
500 0C, show little volume changes on melting and have low vapor pressure in the melt form. For these properties of
the paraffin’s, system-using paraffin’s usually have very long freeze–melt cycle.
b) Non-paraffin’s
The non-paraffin organic are the most numerous of the phase change materials with highly varied properties. Each of
these materials will have its own properties unlike the paraffin’s, which have very similar properties. This is the largest
category of candidate’s materials for phase change storage. Some of the features of these organic materials are as
follows: (i) high heat of fusion, (ii) inflammability, (iii) low thermal conductivity, (iv) low flash points, (v) varying
level of toxicity, and (vi) instability at high temperatures. Fatty acids have high heat of fusion values comparable to
that of paraffin’s. Fatty acids also show reproducible melting and freezing behavior and freeze with no super cooling.
Their major drawback, however, is their cost, which are 2–2.5 times greater than that of technical grade paraffin’s.
They are also mild corrosive. Some fatty acids of interest to low temperature latent heat thermal energy storage
c) Eutectics
A eutectic is a minimum-melting composition of two or more components, each of which melts and freeze congruently
forming a mixture of the component crystals during crystallization . Eutectic nearly always melts and freezes without
segregation since they freeze to an intimate mixture of crystals, leaving little opportunity for the components to
separate. On melting both components liquefy simultaneously, again with separation unlikely.
THERMAL ENERGY STORAGE VIA CHEMICAL REACTIONS –
High energy can be achieved using chemical reactions. Energy may be stored in systems composed of one or more
chemical compounds that absorb or release energy through chemical reactions. There are many forms in which energy
can be stored through chemical reactions. Chemical storage involves an endothermic reversible reaction, which can
be reversed when required to release heat. The chemical produced can often be stored cold (without losses) and can
often be transported easily. For storage of thermal energy in chemical energy, one prefers reversible chemical
reactions. During charging the supplied heat for a chemical reaction to be considered for energy storage, the following
conditions should be met:




The reaction should be run near equilibrium, i.e. reversible.
The reactant, with or without addition of a photosensitizer, should be able to use as much of the solar spectrum
in the terrestrial atmosphere as possible.
The energy stored in the bond energy should be large enough.
The reactants should be cheap.
Table No.3 Various Thermo Chemical Storage options
Reaction
Methane steam reforming
Ammonia dissociation
Thermal dehydrogenation
of metal hydrides
Dehydration of metal
hydroxides
Catalytic dissociation
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Temp. °C
En. density, kJ/kg
CH4+H2O=CO+3H2
2NH3=N2+3H2
MgH2=Mg+H2
480-1195
400-500
250-500
CA(OH)2=CAO+H2O
402-572
6053
3940
3079 heat stor.
9000 H2 stor.
1415
SO3=SO2+ ½O2
520-960
1235
© International Journal of Engineering Sciences & Research Technology
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[Pramod, 4(10): October, 2015]
Technical Performance
Cost
Specific Heat
Required heat exchanger
Required Space
Market Share
Environmental Impact
ISSN: 2277-9655
(I2OR), Publication Impact Factor: 3.785
Table No.4 Comparison of Different Storage Techniques
SHS
LHS
Inexpensive
High
Simple
Large
0.25
Expensive
Medium
Complex
Small
Negligible
Negligible
TCS
Expensive
High
Complex
Large
N/A
CONCLUSION
With the declining supply of conventional sources of energy, the need and demand for the utilization of renewable
sources has increased. With developments in TES technology on almost a day to- day basis, this technology is sure
to become a prominent thermal storage technique in the near future. This review paper is focused on the available
thermal energy storage technology. Those technologies is very beneficial for the humans and as well as for the energy
conservation. This paper presents the current research in this particular field, with the main focus being on the
assessment of the thermal properties of various PCMs.
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Alvaro de Gracia , Luisa F. Cabeza , “Phase change materials and thermal energy storage for buildings”,
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Ercan Ataer, “ Storage Of Thermal Energy” ,UNESCO-EOLSS
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